The goal of this proposal is to study the molecular mechanisms underlying synaptic specificity in the central nervous system (CNS). The model system is the synaptic connections between sensory and motor neurons that form the central portion of the simple stretch reflex. Sensory neurons responsive to muscle stretch (Ia afferents) make strong monosynaptic connections with motoneurons supplying their own muscle (homonymous connections) but weak or no connections with motoneurons supplying antagonistic or functionally unrelated muscles. How these sensory neurons become specified and then recognize their appropriate synaptic partners form the main issues of this study. To pursue these issues, Ia sensory neurons innervating two antagonistic limb muscles will be identified by retrograde labeling and isolated from dorsal root ganglia of chick embryos. cDNA libraries will be made from isolated single sensory neurons and then differentially screened for genes that are expressed in only one neuron type. Identification and characterization of the function of such genes should help us to understand the molecular mechanisms responsible for the formation of specific synapses within the CNS. To this end, the following specific aims are proposed: Specific Aim 1: Isolate single identified Ia sensory neurons and prepare cDNA libraries from these cells. Specific Aim 2: Differentially screen these libraries and clone cell-specific genes in two functionally distinct Ia sensory neurons. Specific Aim 3: Begin the characterization of identified cell-specific genes in two types of Ia sensory neurons using anatomical, biochemical, and electrophysiological techniques.